In recent years, as shown in FIG. 11(A), a production method by use of a rigid core (a) having an outer surface configuration being nearly identical to a tire inner surface configuration of a vulcanized tire (hereinafter called “core methods”) has been proposed. In this core method, tire components such as an inner liner rubber, a carcass ply, a bead core, a belt ply, a sidewall rubber and a tread rubber are sequentially applied on the rigid core (a). This forms an unvulcanized tire (t). The unvulcanized tire (t) is set in a vulcanization mold (b) with the rigid core (a) and vulcanized between the rigid core (a) as an inner mold and the vulcanization mold (b) as an outer mold.
In this core method, it is difficult to employing a structure that both end portions of the carcass ply are turned up around each bead core as a conventional tire. Therefore, the following Patent Document 1 discloses a structure shown in FIG. 11 (B). That is to say, the bead core (d) is divided into axially inner and outer core pieces (d1, d2). And, the both end portions of the carcass ply (c) are held between the inner and outer core pieces (d1 , d2). The inner and outer core pieces (d1, d2) are formed as a helical body formed by helically winding a bead cord ( steel cord) (e) around the tire axis.
However, in case where the tire was formed in the core method, a survey found that the tension of the carcass cord was not enough, and this possibly reduced steering stability, on the ground of this, during vulcanization, binding force of the bead core (d) to the carcass ply (c) became insufficient, the carcass ply (c) possibly moved from between the core pieces (d1, d2) in the loosing direction (radially outwardly). AS a result, even if the heat shrinkage occurs in the carcass cords during the vulcanization, the tension in the carcass cord direction is not sufficiently applied owing to the movement in the loosing direction.
And the results of the inventor's research, it was found that, the twisting direction T of a bead cord (e) and a winding direction R around the tire axis J of the bead cord (e) caused the movement in the loosing direction as shown in FIG. 12 (A). That is, in the conventional core pieces (d1, d2), the winding direction R of the bead cord (e) around the tire axis J was the same, and the bead cord (e) had the same twisting direction T. Therefore, as shown in FIG. 12(B), when the heat shrinkage (f) occurred the carcass cords (c1), in one of the inner and outer core pieces (d1, d2) (the outer core piece d2 in FIG. 12 (B)), the bead cord (e) was rotated in the untwisting direction. Since the steel wire of the cord easily moved in the untwisting direction, the carcass ply (c) easily moved in the heat shrinkage direction, and the tensile force did not subject.